Then pen works fine until you forget to carry a 1 and cause a mess as a result.
Then you get back to technology to fill out the occurrence report which will no doubt be an online form.
There is a reason technology has become such a reliance in aviation. Not such a problem calculating a 1:60 in a GA aircraft.. your visual and can probably get close enough to see the destination.
Doing the math mentally, getting it wrong and not loading enough fuel in the 767 will land you in Gimli.